Flash could be it. It happened on TypePad.com and this URL when I tried to sign in. Also, Joe's Goals when I tried to sign in. No screen shot to send, since there was nothing on the screen. The page was blank, and I had the spinning wheel in the Firefox tab. Interestingly, it didn't time out.

In that case, it's most likely an upstream issue with Firefox. Check to see if there is a submitted bug report of this issue on a Mozilla channel. If you can't find one, you may have found a new bug. As far as I know, the next update to Firefox will be 3.6.4 and will be a substantial update. So it will probably be fixed with the next release, anyway.